1. Field of the Invention
The present invention relates to a vacuum interrupter, more particularly to the vacuum interrupter, an envelope of which includes an improved vacuum-tight brazed seal between an electrical lead rod and another member forming part of the vacuum envelope of the interrupter.
2. Description of the Prior Art
As shown in FIG. 1, the vacuum envelope of a vacuum interrupter generally includes two circular insulating cylinders 1 of glass or alumina ceramics which are coaxially aligned, four metallic sealing rings 2 of Fe-Ni-Co alloy or Fe-Ni alloy, each of which is joined in a vacuum-tight manner to one end of an insulating cylinder 1, two sealing rings 2 at the opposing ends of the insulating cylinders 1 being welded or brazed end-to-end vacuum-tight with a flange 3a of an arc shield 3 sandwiched between the sealing rings 2, two annular end plates 4 and 5 of austenitic stainless steel each welded or brazed vacuum-tight to the sealing rings 2 at opposite ends of the resulting assembly, a stationary electrical lead rod 6 of oxygen-free copper or a copper-based alloy which extends through a central aperture 4a in the end plate 4 in a vacuum-tight manner, a movable electrical lead rod 7 of oxygen-free copper or a copper-based alloy which extends freely through a central aperture 5a in the end plate 5, and a bellows 8 of austenitic stainless steel connecting in a vacuum-tight manner to the end plate 5 and to the movable electrical lead rod 7.
The vacuum-tight brazing is realized in a vacuum brazing process under a high vacuum, the pressure of which is controlled to be 13.3 m Pa (10.sup.-4 Torr) or lower, or in a hermetically brazing process under an inert or reducing atmosphere, the pressure of which is controlled to be about 1.33 to 1333 Pa (10.sup.-2 to 10.sup.1 Torr). A typical brazing metal is a Cu-Ag eutectic. Specifically, in the vacuum brazing process, any of the brazing metals listed in the following Table can be used.
TABLE ______________________________________ Vacuum Brazing Metals and Their Melting Points Vacuum brazing Solidus Liquidus metals temp. temp. No. (wt %) (.degree.C.) (.degree.C.) ______________________________________ 1 61Ag--24Cu--15In 630 685 2 60Ag--27Cu--13In 635 705 3 72Ag--28Cu 779 779 4 20Ag--60Au--20Cu 835 845 5 80Au--20Cu 889 889 6 53Cu--38Mn--9Ni 880 905 7 82Au--18Ni 950 970 8 100Ag 960 960 9 85Ag--15Mn 960 965 ______________________________________
FIG. 2 illustrates a conventional method for vacuum-tight brazing of the bellows 8 to the movable electrical lead rod 7. First, an upper surface of an annular plate 9 formed at the inner end of the bellows 8 abuts a lower surface of a flange 10 being integral part of the movable electrical lead rod 7, a ring of solid brazing metal 11 being placed in contact with the periphery of the flange 10 and the surface of the annular plate 9. Second, the movable electrical lead rod 7 and the bellows 8 are heated to the melting point of the solid brazing metal 11 for hermetically brazing, until the solid brazing metal 11 melts. In cases where the solid brazing metal 11 can easily alloy with copper but not easily alloy with an iron alloy, the resultant molten brazing metal begins deeply diffusing into the copper or copper-based alloy of the flange 10 with its peripheral portion being in contact with the molten brazing metal, resulting in an erodingly diffusing layer of a molten alloy including the brazing metal and copper or copper-based alloy. This alloy of the diffusing layer possesses a melting point lower than that of the copper or copper-based alloy of the movable electrical lead rod 7. The diffusing layer of molten alloy will gradually become a relatively large bulk 12. The bulk 12 of molten alloy shrinks as it solidifies in cooling process, thus generating numerous microcracks therewithin by large contracting. These microcracks will result in many macroscopic cracks 13, which in turn may serve as leak paths in a vacuum-tight sealed portion.